1. Field of the Invention
The present invention relates to an amplifier circuit, and particularly to an amplifier circuit built in a drive circuit for driving a display panel.
2. Description of the Related Art
A display apparatus such as a liquid crystal display apparatus holds input digital display data in a latch circuit, converts the digital display data into an analog display signal by a D/A converter, and then amplifies the analog display signal by an amplifier circuit to drive data lines on a display panel. Generally, a drive voltage for driving the data line is higher than an operation voltage of a logic section such as the latch circuit. For this reason, the breakdown voltages of elements of the D/A converting section need to be higher than the breakdown voltages of elements of the logic circuit section.
A method of increasing the breakdown voltage of an MOS transistor is known in which the length L of a gate is made longer and the thickness Tox of a gate oxide film is made thicker. However, the driving capability of the transistor is reduced in this method. In this case, it is sufficient to increase the width W of the gate to prevent the reduction of the driving capability. However, a circuit area becomes larger exponentially. That is, when the breakdown voltage of the element of the D/A converting section is increased, the circuit area becomes larger.
Here, when a gain of the amplifier circuit is increased, the operation voltage of the D/A converting section can be lowered, thereby making the breakdown voltage of the element lower. Hereinafter, conventional techniques for increasing the gain of the amplifier circuit will be described.
Japanese Laid Open Patent Application (JP-A-Heisei 10-260664: first conventional example) discloses a liquid crystal drive circuit which is connected to a capacitance and includes an amplifier with a gain or amplification factor larger than “1”. Also, Japanese Laid Open Patent Application (JP-A-Heisei 11-184444: second conventional example) discloses a liquid crystal drive circuit which is connected to a resistance and includes an amplifier with a gain or amplification factor larger than “1”.
FIG. 1 is a circuit diagram showing a configuration of an amplifier circuit 1000 in the second conventional example. Referring to FIG. 1, the amplifier circuit 1000 includes an amplifier 80 whose non-inversion input node supplied with a gradation potential Vin and whose inversion input node connected with a connection node between one end of a resistance 81 and one end of a resistance 82. The other end of the resistance 81 is connected to an output node of the amplifier 80. The other end of the resistance 82 is connected to a reference voltage line. Here, it is supposed that the resistance value of the resistance 81 is Rb, the resistance value of the resistance 82 is Ra, the reference potential of the reference voltage line 83 is Vref and the voltage value of an output signal outputted from the amplifier 80 is Vout, an input-output characteristic of the amplifier 80 is expressed by the following equation (1).Vout=(1+Rb/Ra)Vin−(Rb/Ra)Vref  (1)
Here, in case of Vref=0 V, the characteristic is expressed by the following equation (2).Vout=(1+Rb/Ra)Vin  (2)
As described above, gain=(1+Rb/Ra) is determined based on the resistance values of the resistance 81 and resistance 82. For example, in case of the amplifier 80 (ideal gain is 2), in which the resistance 81 and the resistance 82 have a same resistance value as a design value, it is sufficient to output the output signal Vout=0 to 5 V that the gradation voltage in the range of Vin=0 to 2.5 V needs to be supplied to the amplifier 80. Thus, the operation voltage of the D/A converting section supplying the gradation voltage Vin to the non-inversion input node of the amplifier 80 may be a half of a liquid crystal drive voltage. Consequently, since the breakdown voltage of the element can be lowered, the D/A converting section can be reduced in size.
On the other hand, techniques of controlling gain of an amplifier by changing a feedback resistance are described in Japanese Laid Open Patent Applications (JP-P2005-045718A and JP-A-Heisei 7-264003: third and fourth conventional examples).
According to the first and second conventional examples, when the resistance and the capacitance are provided in the amplifier circuit, the gain of the amplifier circuit is determined depending on parameters of these elements. Generally, the number of amplifier circuits for driving data lines on the display panel is a same as the number of data lines. For this reason, the resistances or the capacitances, which are provided in the respective amplifier circuits, vary due to manufacturing variation. That is, when the amplifier circuits described in the first and second conventional examples are used, the gains of the amplifier circuits would be different due to manufacturing variation of the elements built in the amplifier circuits. In detail, referring to FIG. 1, the ratio (Rb/Ra) of the resistance 81 and the resistance 82 varies depending on the amplifier circuit 1000. Since the amplifier circuits respectively drive the data lines based on different display signals, display unevenness would be caused on the display apparatus.
For example, it is supposed that a relative variation of the resistance 81 and the resistance 82 is 1% and Rb=0.99 Ra. In this case, Vout is 1.99 Vin. In the liquid crystal display apparatus, the sensitivity of display unevenness is high at halftone (around Vout=2.5 V). When Vin is 1.25 V, Vout becomes 2.4875 V. As a result, the voltage variation of 12.5 mV arises from the ideal value of 2.5 V. Generally, in the liquid crystal display apparatus, the variation of 5 mV or more in the display signal Vout in the halftone causes the display unevenness. Thus, when the relative variation between the resistance 81 and the resistance 82 is 1%, the display unevenness occurs. That is, when the amplifier circuit 1000 shown in FIG. 1 is used, manufacturing variation needs to be restricted within 0.4% to prevent the display unevenness. It is impractical to restrict resistance variation within 0.4% unless trimming is performed. However, since the driver IC for driving the liquid crystal panel has 384 or more amplification circuits, trimming is difficult.